Easy gallop (6)
I believe the answer is:
'easy gallop' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for canter that I've seen before include "Move at an easy gallop" , "Lope" , "Pace of a horse between trot and gallop" , "Horse gait quicker than 9 across" , "(Horse's) pace between a trot and a gallop" .)